Electric wire coatings, tubes, especially heat-shrinkable tubes, and the like are formed from polyolefin resins because of the excellent processability, chemical resistance, pliability, and electrical characteristics of the resins. Since polyolefin resins in themselves are a combustible material, flame retardancy is imparted to the resins by incorporating thereinto a flame retardant and a flame retardation aid.
In addition to the flame retardancy, thermal stability is also required of electric wires and tubes.
Decabromodiphenyl ether is known as a flame retardant having properties of imparting both flame retardancy and thermal stability. However, this flame retardant has a problem that it migrates to the resin surface to form a bloom with the lapse of time, thereby to considerably impair the appearance of the product.
In order to overcome the above problem, JP-A-1-153742 discloses a flame-retardant tube to which flame retardancy has been imparted by the combined use of ethylenebis(tetrabromophthalimide) and dodecachlorododecahydrodibenzocyclooctene. The tube is excellent in flame retardant properties and thermal stability and free from flame retardant blooming. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
In the case where a high degree of flame retardancy is to be obtained, a large amount of a flame retardant should be incorporated. However, there has been a problem that if such flame retardants as ethylenebis(tetrabromophthalimide) and dodecachlorododecahydrodibenzocyclooctene are added to a resin in a large amount, the thermal stability of the resin is impaired.